Fibroids, tumors and other tissue masses are often treated by ablation. In many cases, local ablation of the diseased tissue is carried out by inserting a therapeutic device into the tissue and carrying out therapeutic activity designed to destroy the diseased cells. For example, electrical energy (usually alternative current of radio frequency—RF) may be applied to the affected area by placing one or more electrodes into the affected tissue and discharging electric current therefrom to ablate the tissue. Alternatively, tissue may be ablated cryogenically, by applying heat or chemically by injecting fluids with appropriate properties to the target tissue.
When electrical energy is used, the size and shape of the region of tissue ablated depends, in part, on the configuration of the electrodes used for the procedure and on the strength of the charge applied. The electrical energy dissipates very rapidly with distance from the electrodes, it has been difficult to maintain desired levels of energy density within large volumes of tissue. Therefore, the ablation of larger target tissue masses has often necessitated repeated application of the ablation electrodes at multiple locations within each target tissue mass. This repetition increases the complexity, duration and cost of these procedures.
In addition, the shapes and sizes of lesions formed by existing RF ablation systems often do not reflect the shapes of the target tissue masses. For example, tumors are often generally spherical and some of them are quite large. The shapes of tissue masses ablated by conventional monopolar ablation systems are generally spherical, but the tissue masses affected are small, while conventional bipolar ablation methods produce bigger thermal mass, but ablate substantially cylindrical shapes. In both these cases, repeated applications and the ablation of substantial amounts of non-targeted tissue may be necessary to achieve a desired degree of ablation throughout an entire target tissue mass.